Although porous, fibrous fluorocarbon structures are used in a variety of applications, the demands placed upon such structures are particularly severe in certain filtration applications. For example, filter structures may be exposed to very hot liquids which may be highly acidic or strongly caustic. Fluorocarbon polymers, particularly polytetrafluoroethylene, are, in large measure, unaffected by such conditions, but conventional binders are not. Accordingly, if conventional binders are used to form fibrous fluorocarbon structures, once the binder decomposes, the structural integrity of the fibrous structure is destroyed.
Fluorocarbon polymer resins are not readily dispersed, and conventional dispersion methods do not produce a fiber dispersion sufficiently uniform or stable to produce a laydown with a uniform distribution of fibers. Additionally, fluorocarbon resins do not readily adhere to fluorocarbon fibers. It is difficult, therefore, to ensure that adequate amounts of binder remain in the laydown after drainage and drying when the binder is admixed into the fiber dispersion. Further, it is difficult to provide uniform distribution of the binder, throughout the structure, and, concomitantly, a structure with uniform strength.
Fluorocarbon polymers also tend to shrink when heated. This shrinkage tends to exacerbate the problem of poor structural integrity of the formed structures due to inadequate amounts and non-uniform distribution of binder in the laydown since, upon shrinking of the individual fibers, the structure tends to be disrupted and pulled apart.
This invention is directed to porous, fibrous fluorocarbon structures comprising polytetrafluoroethylene fibers and a fluorocarbon binder, which structures have increased structural integrity and enhanced and more uniform properties by virtue of a more uniform distribution of fiber and binder. It is directed further to such structures which are suitable for use as a support or a drainage layer in composite filter structures. It also is directed to a method for forming such structures.